Motorcycle frame structure

ABSTRACT

A motorcycle frame structure includes pivot plates at rear portions of left and right main frames extending rearwardly and downwardly from a head pipe. A seat rail is fastened to an upper portion of the pivot plate at two positions which are vertically spaced apart from each other, and also at the front end of the seat rail. A first attachment portion and a second attachment portion are attached to a front end of the seat rail. The first attachment portion is fastened to the pivot plate from the horizontally external side of the seat rail with a first bolt, and laterally overlaps a rear upper lateral surface of the pivot plate. The second attachment portion is fastened to the pivot plate from the rear side of the pivot plate with a second bolt, and overlaps a rear portion of the pivot plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 USC 119 based on Japanesepatent application No. 2008-092854, filed on Mar. 31, 2008. The entiresubject matter of this priority document, including specification,claims and drawings, is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a motorcycle frame structure. Moreparticularly, the present invention relates to a body frame of amotorcycle in which a pivot plate, vertically and swingably supporting afront portion of a swing arm which is rotatably supporting a rear wheelat a rear portion, is provided at a rear portion of each of a pluralityof main frames extending rearward and downward from a head pipe which issteerably supporting a front fork and a steering handlebar, and a frontend of a seat rail is fastened to an upper portion of the pivot plate attwo positions of a front end vertically spaced apart from each other.

2. Background Art

A conventional motorcycle body frame is known in which two upper andlower front ends of the seat rail are laterally put on the rear portionof a pivot plate and are fastened with a bolt to the pivot plate fromthe left-right direction outside of the body frame. A frame of thisknown design is described in Japanese Laid-open Patent No. 2003-81166.

Incidentally, the fastened portion of the seat rail to the pivot plateneeds strength to withstand a load applied to the seat rail from above.As disclosed in Japanese Laid-open Patent No. 2003-81166 mentionedabove, the configuration in which the two upper and lower front ends ofthe seat rail are fastened to the pivot plate from the left-rightdirection outside of the body frame needs to increase the rigidity of aboss attached to the pivot plate in order to ensure the strength of sucha fastened portion. This increases the thickness of the attachment boss,which leads to an increase in the weight of the pivot plate.

Although the known motorcycle frame has some utility for its intendedpurposes, a need still exists in the art for an improved motorcycleframe structure. In particular, there is a need for an improvedmotorcycle frame structure in which a front end of a seat rail canfirmly be fastened to a pivot plate while preventing an increase in theweight of the pivot plate.

SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention, to provide amotorcycle frame structure in which a front end of a seat rail canfirmly be fastened to a pivot plate while preventing an increase in theweight of the pivot plate. To achieve the foregoing object, according toa first aspect and feature of the present invention, a motorcycle framestructure is provided in which a pivot plate, vertically and swingablysupporting a front portion of a swing arm which is rotatably supportinga rear wheel at a rear portion, is provided at a rear portion of each ofa plurality of left and right main frames extending from a head pipewhich is steerably supporting a front fork and a steering handlebar anda front end of a seat rail is fastened to two positions of an upperportion of the pivot plate, the two positions being vertically spacedapart from each other, a first attachment portion laterally overlappinga rear upper lateral surface of the pivot plate and a second attachmentportion disposed below the first attachment portion to overlap a rearsurface of a rear portion of the pivot plate from the rear are attachedto the front end of the seat rail, and the first attachment portion isfastened to the pivot plate from the horizontally external side of theseat rail with a first bolt, and the second attachment portion isfastened to the pivot plate from the rear side of the pivot plate with asecond bolt.

According to the above arrangement, the second attachment portion isdisposed below the first attachment portion to overlap the rear surfaceof the rear portion of the pivot plate from the rear is fastened to thepivot plate from the rear side of the pivot plate with the second bolt.Therefore, a load applied to the second bolt resulting from a loadapplied to the seat rail from above becomes extremely small. Thus, thepivot plate needs only to be provided with a simple screw hole adaptedto receive the second bolt screwed thereinto, at a portion correspondingto the second attachment portion. Consequently, it is possible to firmlyfasten the front end of the seat rail to the pivot plate whilepreventing a weight increase due to the increased thickness of the pivotplate at a portion corresponding to the second attachment portion of theseat rail.

According to a second feature and aspect of the present invention, inaddition to the first aspect and feature of the present invention, thepivot plates are provided with a pair of respective left and rightto-be-attached portions to which a pair of the first left and rightattachment portions are respectively attached, and with a cross memberconnecting both the to-be-attached portions, and the cross member isformed with a cushion support portion supporting an upper end portion ofthe rear cushion.

In the above arrangement, the cushion support portion sufficientlyensuring strength and rigidity and the pair of left and rightto-be-attached portions can be commoditized to ensure the attachmentportion rigidity of the seat rail to the pivot plate without increasingweight.

According to a third aspect and feature of the present invention, inaddition to the first and second aspects and features, the front end ofthe seat rail is formed to externally overlap a rear portion of thepivot plate as viewed from the side.

In the above arrangement, the front end of the seat rail externallyoverlaps the rear portion of the pivot plate as viewed from the side.Therefore, the horizontal attachment rigidity of the seat rail to thepivot plate is improved and the pivot plate can be looked thin relativeto the seat rail to enhance external appearance.

According to a fourth aspect and feature of the present invention, inaddition to any of the first through third aspects and features, a pairof the left and right seat rails are joined together to constitute aseat rail assembly, and the seat rail assembly is formed with a box-likeportion disposed at a position connecting a pair of the left and rightsecond attachment portions together when the seat rail assembly isviewed from the rear.

In the above arrangement, the pair of left and right seat rails arejoined together to constitute the seat rail assembly, and the seat railassembly is formed with the box-like portion disposed at a positionconnecting a pair of the left and right second attachment portions whenthe seat rail assembly is viewed from the rear. Therefore, a loadapplied to the second attachment portions resulting from a twist in theseat rails can be reduced by action of the box-like portion to reduce aload applied to the pivot plate, thereby further reducing the weight ofthe pivot plate.

For a more complete understanding of the present invention, the readeris referred to the following detailed description section, which shouldbe read in conjunction with the accompanying drawings. Throughout thefollowing detailed description and in the drawings, like numbers referto like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral view of a motorcycle according to an exemplaryembodiment of the present invention.

FIG. 2 is an enlarged lateral view of a major portion of FIG. 1 with abody cover omitted.

FIG. 3 is a perspective view illustrating a head pipe, main framesections, and pivot plates according to an exemplary embodiment of thepresent invention.

FIG. 4 is a lateral view illustrating the head pipe, main frame sectionsand pivot plates according to an exemplary embodiment of the presentinvention.

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 4.

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 4.

FIG. 7 is a perspective view of a seat rail assemble according to anexemplary embodiment of the present invention.

FIG. 8 is a perspective view as viewed from arrow 8 of FIG. 7.

FIG. 9 is a lateral view of an air cleaner according to an exemplaryembodiment of the present invention.

FIG. 10 is a perspective view as viewed from arrow 10 of FIG. 9.

FIG. 11 is a cross-sectional view taken along line 11-11 of FIG. 9.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

A selected exemplary embodiment of the present invention will now bedescribed, with reference to the drawings. Throughout this description,relative terms like “front”, “rear”, “longitudinal,” “crosswise,”“vertical,” and the like are used in reference to a vantage point of anoperator of the vehicle, seated on the driver's seat and facing forward.It should be understood that these terms are used for purposes ofillustration, and are not intended to limit the invention.

Referring first to FIG. 1, for example, a 4-cylinder V-type engine E ismounted on a vehicle frame F of a motorcycle. A rear wheel WR isrotatably supported by the rear portion of a swing arm 15 verticallyswingably carried by the body frame F. Power of the engine E istransmitted to the rear wheel WR via a drive shaft 16 extending back andforth. A portion of the engine E and the body frame F are covered by abody cover 17, which is mounted to the body frame F.

With additional reference to FIGS. 2 to 4, the body frame F includes ahead pipe 20 steerably supporting a front fork 18 rotatably supporting afront wheel WF and a steering handlebar 19; a pair of left and rightmain frame sections 21 extending rearward and downward from the headpipe 20; a pair of left and right pivot plates 22 connected to the rearportions of the main frame sections 21 and extending downward; and apair of left and right seat rails 23 connected at front ends to theupper portions of the pivot plates 22 and extending rearward upward. Therear wheel WR is rotatably supported by the rear portion of the swingarm 15 swingably supported at a front end by the pivot plates 22. Inaddition, a link mechanism 24 is provided between the lower portion ofone of both the pivot plates 22 and the front portion of the swing arm15. A rear cushion unit 26 is provided between the body frame F and alink member 25 constituting part of the link mechanism 24.

The main frame sections 21 are provided integrally with engine hangers27 hanging downward from intermediate portions of the main framesections 21. An engine main body 32 of the engine E is suspended by andmounted to the engine hangers 27 and the pivot plates 22. Specifically,first and second engine support portions 28 and 29 are integrallyprovided at the lowermost portions and rear upper portions,respectively, of the engine hangers 27. In addition, third and fourthengine support portions 30 and 31 are integrally provided at the upperportions and lower portions, respectively, of the pivot plates 22. Theengine main body 32 is supported by the engine support portions 28, 29,30, 31.

Referring to FIGS. 5 and 6, the main frame sections 21 are each formedhollow by joining together two frame members 33, 34 subjected to pressworking. The front ends of the main frame sections 21 are welded to thehead pipe 20. Similarly, also the engine hangers 27 integral with themain frame sections 21 are each formed hollow by joining together thetwo frame members 33, 34. The pivot plates 22 are welded to the rearportions of the pair of main frame sections 21 and extend vertically.The upper portions of both the pivot plates 22 are integrally connectedto each other by an upper cross member 35. The lower portions of boththe pivot plates 22 are integrally connected to each other by a lowercross member 36.

The engine hangers 27 are each integrally continuous with anintermediate portion of the main frame 21 to extend downwardlytherefrom. A front edge 27 a of the engine hanger 27 is formed to beinclined forwardly and upwardly toward the head pipe 20. In addition,both frame members 33, 34 of the main frame sections 21 are provided atupper portions of the engine hangers 27 with through-holes 37 to reducethe weight of the main frame sections 21. The main frame sections 21 areeach provided with a reinforcing portion 38 which extends generallyparallel to the front edge 27 a of the engine hanger 27 so as tolongitudinally traverse the through-hole 37. The second engine supportportion 29 is provided on the extension of the reinforcing portion 38and at the lower portion of the engine hanger 27 in order to carry theengine main body 32.

A plurality of, e.g. three, attachment bosses 39A, 39B, 39C are providedon each of the main frame sections 21 including the engine hangers 27 inorder to attach a vehicle constituting component, e.g., the body cover17 thereto. Ribs 40A, 40A are integrally provided on the inner surfacesof the two frame members 33, 34 constituting the main frame sections 21and the engine hangers 27. As shown in FIG. 6, each of the ribs 40A, 40Aconnect the first engine supporting portion 28 located at the lowermostportion of the engine hanger 27, the attachment bosses 39A, 39B and thehead pipe 20 together. In addition, the rib 40B connecting theattachment boss 39B with the head pipe 20 and the rib 40C connecting theattachment boss 39C with the head pipe 20 are integrally provided closeto the main frame sections 21 and on the inner surfaces of both theframe members 33, 34. The ribs 40A, 40B and 40C can increase therigidity of the main frame sections 21 and engine hangers 27 per se andreduce vibration sound transmitted from the body cover 17 to the mainframe sections 21 and to the engine hangers 27.

In addition, as illustrated in FIG. 6, each of the attachment bosses39A, 39B, 39C is provided between the rear and front frame members 33,34 of each of the main frame sections 21 and of each of the enginehangers 27.

Incidentally, the upper cross member 35 integrally connecting therespective upper portions of both the pivot plates 22 is formed at acentral portion with a cushion support portion 35 a for supporting theupper end of the rear cushion 26. In addition, the cushion supportportion 35 a is provided with an insertion hole 43. In this way, thisinsertion hole 43 receives an upper portion of a bolt 42 passedtherethrough. The bolt 42 is engaged from below with the cushion supportmember 41 connected to the upper end of the rear cushion unit 26 to passthrough the cushion support member 41. A nut 44 is screwed to theportion of the bolt 42 projecting upward from the insertion hole 43.Thus, the upper end of the rear cushion unit 26 is connected to andsupported by the upper cross member 35 of the body frame F.

The seat rails 23 are each fastened at a front end portion to the upperportion of the pivot plate 22 at two positions vertically spaced apartfrom each other. The front end portion of each seat rail 23 is providedwith a first attachment portion 23 a and a second attachment portion 23b. The first attachment portion 23 a is provided to laterally overlap arear upper lateral surface of the pivot plate 22. The second attachmentportion 23 b is disposed below the first attachment portion 23 a tooverlap from rearward the rear surface of the rear portion of the pivotplate 22.

On the other hand, the pivot plates 22 are each provided at rear upperportions with attachment bosses 47 adapted to each receive a first bolt45 screwed thereinto, the first bolt 45 being inserted into the firstattachment portion 23 a from the horizontal outside of the body frame F.In addition, both the bosses 47 are connected to each other by the uppercross member 35. The pivot plates 22 are each provided at a rear surfaceof the rear portion with a screw hole 48 adapted to receive a secondbolt 46 screwed thereinto, the second bolt 46 being inserted into eachof the second attachment portions 23 b from the rear side of the bodyframe F.

With additional reference to FIGS. 7 and 8, the seat rail 23 integrallyincludes an upper frame portion 23 c, a lower frame portion 23 d, a rearframe portion 23 e and a thin wall portion 23 f. The upper frame portion23 c is provided with a first attachment portion 23 a at a front end andextends rearward and upward. The lower frame portion 23 d is providedwith a second attachment portion 23 b at a front end, extends rearwardand upward, and merges with the intermediate portion of the upper frameportion 23 c. The rear frame portion 23 e bends inwardly from the rearend of the upper frame 23 c. The thin wall portion 23 f connects thefront portion of the upper frame portion 23 c with the lower frameportion 23 d. Both the seat rails 23, 23 are joined together toconstitute a seat rail assembly 75.

The seat rail assembly 75 is formed with a box-like portion 76 disposedat a position connecting a pair of the left and right second attachmentportions 23 b when the seat rail assembly 75 is viewed from the rear.The box-like portion 76 is formed by joining together box-like halfportions 23 g each provided integrally with the lower frame portion 23 dof the seat rail 23 and hanging downward. The box-like portion 76 opensforward and upward and houses, e.g. an ABS modulator, etc.

Additionally, the front ends 23 i of the seat rails 23 are formed tooverlap the rear portion of the pivot plate 22 with the first and secondattachment portions 23 a, 23 b fastened to the pivot plate 22.

As shown in FIG. 2, the engine main body 32 is formed as a V-typewater-cooled one having a front bank BF located anteriorly and a rearbank BR located rearward of the front bank BF while mounted on themotorcycle. A crankshaft 51 extending in a vehicle-widthwise direction,i.e., in a left-right direction of the motorcycle is rotatably journaledby the crankcase 50 shared by both the banks BF, BR.

The crankcase 50 is formed by joining together an upper case half-body50 a and a lower case half-body 50 b. The front bank BF and the rearbank BR are formed integrally with the upper case half-body 50 a so asto be V-shaped in the back and forth direction of the body frame F. Anaxis of the crankshaft 51 is located on a joint surface between theupper case half-body 50 a and the lower case half-body 50 b.

The front bank BF includes a front cylinder block 52F integrallycontinuous with the upper case half-body 50 a of the crankcase 50; afront cylinder head 53F joined to the front cylinder block 52F; and afront head cover F joined to the front cylinder head 53F. The rear bankBR includes a rear cylinder block 52R integrally continuous with theupper case half-body 50 a of the crankcase 50; a rear cylinder head 53joined to the rear cylinder block 52R; and a rear head cover 54R joinedto the rear cylinder head 53R.

A single air cleaner 56 shared by the front and rear banks BF, BR isdisposed above the engine main body 32. A pair of left and right runningair introduction ducts 57 opening forward are respectively joined toboth left and right sides of the air cleaner 56. A fuel tank 55 ismounted on both the main shafts 21 of the body frame F so as to coverthe air cleaner 56 and the running air introduction ducts 57 from above.As shown in FIGS. 7 and 8, respective support arm portions 23 h forsupporting the rear portion of the fuel tank 55 are integrally providedon both the main frames 23 to extend upward from the corresponding frontend portions of the upper frames 23 c.

Referring to FIGS. 9 to 11, the air cleaner 56 includes a cleaner case60 and a pair of cleaner elements 61, 61. The cleaner case 60 is formedin both left and right lateral walls with introduction ports 58communicating with the corresponding running air introduction ducts 57.The cleaner elements 61, 61 are fixedly disposed in the cleaner case 60to partition the inside thereof into an unfiltered air chamber 62communicating to both the introduction ports 58 and a filtered airchamber 63 disposed below the unfiltered air chamber 62 and shared bythe front bank BF and the rear bank BR.

The cleaner case 60 includes a lower case 64 forming a lateral wall ofthe filtered air chamber 63 and an upper case 65 detachably connected tothe lower case 64 to form the unfiltered air chamber 62 between thelower case 64 and the upper case 65. The lower case 64 includes a firstcase member 66 formed like a bowl to open upward and a second casemember 67 connected from above to the first case member 66 to form thefiltered air chamber 63 between the first case member 66 and the secondcase member 67. A ceiling wall 67 a of the second case member 67 isformed in a mountain-shape to grow in height toward the vertical PLincluding a vehicle-body centerline extending forward and rearwardthrough the widthwise central portion of the cleaner case 60.

The ceiling wall 67 a of the second case member 67 is formed withopening portions 68, 68 on both sides of the vertical PL. The cleanerelements 61 are attached to the ceiling wall 67 a of the second casemember 67 so as to bury the corresponding openings 68. Thus, both thecleaner elements 61 are fixedly housed inside the cleaner case 60 so asto be inclined to be gradually lowered as they are away from thevertical PL.

Both the running air introduction ducts 57 are each formed by joiningtogether a duct half-portion 67 b integrally continuous with the secondcase member 67 of the lower case 64 and a duct half-portion 65 aintegrally continuous with the upper case 65. The running airintroduction ducts 57 extend forwardly and obliquely downwardly throughboth sides of the cleaner case 60 from introduction ports 58 provided onboth lateral walls of the upper case 65 of the cleaner case 60 so as tobe continuous with the unfiltered air chamber 62.

In this way, the cleaner elements 61 disposed on an incline which isgradually lowered as they are away from the vertical PL on both thesides of the vertical PL including the vehicle body centerline extendingforward and rearward are made to face the introduction ports 58 of boththe lateral walls of the cleaner case 60.

Further, a partition wall 69 is integrally provided on the upper innersurface of the upper case 65 at a position corresponding to between thepair of cleaner elements 61 to partition the inside of the unpurified 62into the left and right portions corresponding respectively to both therunning air introduction ducts 57.

Referring again to FIG. 2, throttle bodies 70F individuallycorresponding to two cylinders provided for the front bank BF areconnected to the lateral wall, facing the rear bank BR, of the frontcylinder head 53F in the front bank BF so as to rise upward. Inaddition, throttle bodies 70R individually corresponding to twocylinders provided for the rear bank BR are connected to the lateralwall, facing the front bank BF, of the rear cylinder head 53R in therear bank BR so as to rise upward. Fuel injection valves 71F, 71R areeach attached to a corresponding one of the throttle bodies 70F, 70R.

Referring to FIGS. 8 and 9, a pair of intake pipes 72F, 72F connectedrespectively to both the throttle bodies 70F of the front bank BF and apair of intake pipes 72R, 72R connected respectively to both thethrottle bodies 70R are attached to the bottom wall of the lower case64, i.e., of the first case member 66 included in the cleaner case 60 ofthe air cleaner 56 in such a manner that their upper ends open towardthe filtered air chamber 63.

A description is next given of the operation of the present exemplaryembodiment. The engine main body 32, having the front and rear banks BF,BR which are V-shaped in the back and forth direction of the body frameF, is mounted on the body frame F. The single air cleaner 65 shared bythe front and rear banks BF, BR includes the cleaner case 60 formed inboth the left and right lateral walls with the introduction ports 58communicating with the pair of corresponding left and right running airintroduction ducts 57 opening forward, and the pair of cleaner elements61, 61 fixedly disposed in the cleaner case 60 to partition the insidethereof into the unfiltered air chamber 62 communicating with both theintroduction ports 58 and the filtered air chamber 63 disposed below theunfiltered air chamber 62 and shared by the front and rear banks BF, BR.Both the cleaner elements 61 are fixedly housed in the cleaner case 60so as to be disposed on both the sides of the vertical PL including thevehicle body centerline extending in the back and forth direction and tobe inclined to be gradually lowered as they are away from the verticalPL to face both the corresponding introduction ports 58.

Thus, running air introduced into the unfiltered air chamber 62 from thepair of left and right running air introduction ports 57 hits both thecleaner elements 61, which are equally shaped, passes through thecorresponding cleaner elements 61, and flows into the filtered airchamber 63. Since the air flowing in the filtered air chamber 63 isequalized, intake air can equally be distributed into the front and rearbanks BF, BR. In addition, since both the cleaner elements 61 arearranged on an incline, the filtration area can be increased withoutincreasing the width of the cleaner case 60.

The cleaner case 60 includes the lower case 64 forming the lateral wallof the filtered air chamber 63 and the upper case 65 detachablyconnected to the lower case 64 to form the unfiltered air chamber 62between the lower case 64 and the upper case 65. In addition, thepartition wall 69 is integrally provided on the upper inner surface ofthe upper case 65 at a position corresponding to between both thecleaner elements 61 to partition the inside of the unpurified 62 intothe left and right portions corresponding respectively to both therunning air introduction ducts 57. Thus, running air introduced fromboth the running air introduction ducts 57 into the unfiltered airchamber 62 evenly passes through the cleaner elements 61 and flowstoward the filtered air chamber 63.

In the body frame F, the pair of left and right main frame sections 21extending rearward and downward from the head pipe 20 are eachintegrally provided with the engine hanger 27 having the front edge 27 aformed to be inclined forward and upward toward the head pipe 20 andhanging downward from the intermediate portion of the main frame 21. Inaddition, both the main frame sections 21 are each formed with thethrough-hole 37 above the engine hanger 21. Thus, weight saving of themain frame sections 21 can be achieved.

In addition, the reinforcing portions 38 extending generally parallel tothe front edges 27 a of the engine hangers 27 are each provided in themain frame 21 so as to longitudinally traverse the through-hole 37.Therefore, the through-holes 37 can each effectively be reinforced bythe reinforcing portion 38 to withstand a bending load acting on themain frame 21 with the head pipe 20 serving as a fulcrum.

The first engine support portions 28 for supporting the engine main body32 are each provided at the lowermost portion of the engine hanger 27formed hollow along with the main frame 21. The ribs 40A, 40B connectingthe attachment bosses 39A, 39B, the first engine support portion 28 andthe head pipe 20 together are provided to project from the internalsurfaces of the engine hanger 27 and the main frame 21, respectively.The attachment bosses 39A and 39B are provided on both the main frame 21and the engine hanger 27, respectively, to mount the body cover 17thereto in this exemplary embodiment. Incidentally, the attachment boss39A or 39B may be provided on at least a corresponding one of the mainframe 21 and the engine hanger 27. Thus, the rigidity of the main framesections 21 and of the engine hangers 27 can be ensured and the externalappearance thereof can be enhanced because the ribs 40A, 40B will not beexposed to the external surfaces of the main frame sections 21 and ofthe engine hangers 27.

The attachment bosses 39A, 39B, 39C are provided between the rear andfront frame members 33, 34 of at least one of the main frame 21 and theengine hanger 27. Thus, the attachment bosses 39A, 39B, 39C are made tofunction as struts of the hollow main frame 21 and engine hanger 27 tothereby enhance the rigidity of the main frame 21 and of the enginehanger 27.

The second engine support portions 29 are each provided on the extensionof the reinforcing portion 38 and below the engine hanger 27; therefore,the supporting rigidity of the engine main body 32 can be enhanced.

The front end portion 23 i of the seat rail 23 is fastened at two upperand lower positions to the upper portion of the pivot plate 22. In thiscase, the first attachment portion 23 a laterally overlapping a rearupper lateral surface of the pivot plate 22 and the second attachmentportion 23 b disposed below the first attachment portion 23 a to overlapfrom rearward the rear surface of the rear portion of the pivot plate 22are provided at the front end portion 23 i of the seat rail 23. Thefirst attachment portion 23 a is fastened to the pivot plate 22 with thefirst bolt 45 from the horizontal outside of the seat rail 23 and thesecond attachment portion 23 b is fastened to the pivot plate 22 withthe second bolt 46 from the rear side of the pivot plate 22.

With such an arrangement, a load applied to the second bolt 46 alongwith the load applied from the seat rail 23 from above is extremelysmall; therefore, the pivot plate 22 needs only to be provided with asimple screw hole 48 adapted to receive the second bolt 46 screwedthereinto, at a portion corresponding to the second attachment portion23 b. Thus, it is possible to firmly fasten the front end portion of theseat rail 23 to the pivot plate 22 while preventing a weight increasedue to the increased thickness of the pivot plate 22 at a portioncorresponding to the second attachment portion 23 b of the seat rail 23.

The pivot plates 22 are provided with the attachment bosses 47 to whichthe pair of left and right attachment portions 23 a are attached suchthat the upper cross member 35 connects both the bosses 47. The uppercross member 35 is provided with the cushion support portion 35 a forsupporting the upper end of the rear cushion 26. Therefore, the cushionsupport portion 35 a sufficiently ensures the strength and rigidity andthe pair of left and right bosses 47 can be commoditized to therebyensure rigidity of the attachment portions of the seat rails 23 to thepivot plates 22 without increasing the weight.

The front end portions 23 i of the seat rails 23 externally overlap therear portion of the pivot plate 22 as viewed from the side. Therefore,the horizontal attachment rigidity of the seat rails 23 to the pivotplates 22 is increased and the pivot plate 22 can thin relative to theseat rails 23, thereby enhancing the external appearance.

Further, the pair of left and right seat rails 23 are joined together toconstitute the seat rail assembly 75. The seat rail assembly 75 isformed with the box-like portion 76 disposed at a position connecting apair of the left and right second attachment portions 23 b when the seatrail assembly 75 is viewed from the rear. A load applied to the secondattachment portions 23 b resulting from a twist in the seat rails 23 canbe reduced by action of the box-like portion 76 to reduce a load appliedto the pivot plate 22, thereby further reducing the weight of the pivotplate 22.

Although the present invention has been described herein with respect toa number of specific illustrative embodiments, the foregoing descriptionis intended to illustrate, rather than to limit the invention. Thoseskilled in the art will realize that many modifications of theillustrative embodiment could be made which would be operable. All suchmodifications, which are within the scope of the claims, are intended tobe within the scope and spirit of the present invention.

1. A motorcycle frame structure, comprising: a head pipe pivotallysupporting a front fork and a steering handlebar operatively attached tothe front fork; a pair of left and right main frame sections extendingrearwardly and downwardly from the head pipe; a pair of left and rightpivot plates provided at respective rear portions of the main framesections and configured to reciprocally swingably support a frontportion of a swing arm therebetween, said swing arm rotatably supportinga rear wheel at a rear portion thereof; and a seat rail fastened at afront end of an upper portion of the pivot plates in two positions, thetwo positions being vertically spaced apart from each other, wherein afirst attachment portion laterally overlapping a rear upper lateralsurface of the pivot plates and a second attachment portion disposedbelow the first attachment portion to overlap a rear surface of a rearportion of the pivot plates from the rear are attached to the front endof the seat rail, and the first attachment portion is fastened to thepivot plates from the horizontally external side of the seat rail with afirst bolt, and the second attachment portion is fastened to the pivotplates from the rear side of the pivot plates with a second bolt.
 2. Themotorcycle frame structure according to claim 1, wherein: the pivotplates are provided with a pair of respective left and right bosses towhich the first attachment portion is respectively attached, and withthe cross member connecting both the bosses, and the cross member isformed with a cushion support portion supporting an upper end portion ofthe rear cushion.
 3. The motorcycle frame structure according to claim1, wherein the front end of the seat rail is formed to externallyoverlap a rear portion of the pivot plates as viewed from the side. 4.The motorcycle frame structure according to claim 2, wherein the frontend of the seat rail is formed to externally overlap a rear portion ofthe pivot plates as viewed from the side.
 5. The motorcycle framestructure according to claim 1, wherein a pair of the left and rightseat rails are joined together to constitute a seat rail assembly, andthe seat rail assembly is formed with a box-like portion disposed at aposition connecting a pair of the left and right second attachmentportions when the seat rail assembly is viewed from the rear.
 6. Themotorcycle frame structure according to claim 2, wherein a pair of theleft and right seat rails are joined together to constitute a seat railassembly, and the seat rail assembly is formed with a box-like portiondisposed at a position connecting a pair of the left and right secondattachment portions when the seat rail assembly is viewed from the rear.7. The motorcycle frame structure according to claim 3, wherein a pairof the left and right seat rails are joined together to constitute aseat rail assembly, and the seat rail assembly is formed with a box-likeportion disposed at a position connecting a pair of the left and rightsecond attachment portions when the seat rail assembly is viewed fromthe rear.
 8. The motorcycle frame structure according to claim 4,wherein a pair of the left and right seat rails are joined together toconstitute a seat rail assembly, and the seat rail assembly is formedwith a box-like portion disposed at a position connecting a pair of theleft and right second attachment portions when the seat rail assembly isviewed from the rear.
 9. The motorcycle frame structure of claim 1further including, an engine hanger provided integrally with each of therespective left and right main frame sections and configured to extenddownwardly from corresponding intermediate portions of the left andright main frame sections.
 10. A motorcycle frame structure, comprising:a head pipe pivotally supporting a front fork and a steering handlebaroperatively attached to the front fork; a pair of left and right mainframe sections extending rearwardly and downwardly from the head pipe; apair of left and right pivot plates provided at respective rear portionsof the main frame sections and configured to reciprocally swingablysupport a front portion of a swing arm therebetween, said swing armrotatably supporting a rear wheel at a rear portion thereof; and anengine hanger provided integrally with each of the respective left andright main frame sections and configured to extend downwardly fromcorresponding intermediate portions of the left and right main framesections; an engine support portion provided at a lowermost portion ofthe engine hanger and cooperating with the left and right main framesections to support an engine main body; attachment bosses provided onat least one of the main frame sections and the engine hangers, and ribsconnecting the engine support portion with the head pipe, wherein saidribs are provided projecting inwardly on respective inner surfaces ofthe main frame sections and the engine hangers; and a seat rail fastenedat a front end of an upper portion of the pivot plates in two positions,the two positions being vertically spaced apart from each other, whereina first attachment portion laterally overlapping a rear upper lateralsurface of the pivot plates and a second attachment portion disposedbelow the first attachment portion to overlap a rear surface of a rearportion of the pivot plates from the rear are attached to the front endof the seat rail, and the first attachment portion is fastened to thepivot plates from the horizontally external side of the seat rail with afirst bolt, and the second attachment portion is fastened to the pivotplates from the rear side of the pivot plates with a second bolt. 11.The motorcycle frame structure according to claim 10, wherein: the pivotplates are provided with a pair of respective left and right bosses towhich the first attachment portion is respectively attached, and withthe cross member connecting both the bosses, and the cross member isformed with a cushion support portion supporting an upper end portion ofthe rear cushion.
 12. The motorcycle frame structure according to claim10, wherein the front end of the seat rail is formed to externallyoverlap a rear portion of the pivot plates as viewed from the side. 13.The motorcycle frame structure according to claim 11, wherein the frontend of the seat rail is formed to externally overlap a rear portion ofthe pivot plates as viewed from the side.
 14. The motorcycle framestructure according to claim 10, wherein a pair of the left and rightseat rails are joined together to constitute a seat rail assembly, andthe seat rail assembly is formed with a box-like portion disposed at aposition connecting a pair of the left and right second attachmentportions when the seat rail assembly is viewed from the rear.
 15. Themotorcycle frame structure according to claim 11, wherein a pair of theleft and right seat rails are joined together to constitute a seat railassembly, and the seat rail assembly is formed with a box-like portiondisposed at a position connecting a pair of the left and right secondattachment portions when the seat rail assembly is viewed from the rear.16. The motorcycle frame structure according to claim 11, wherein a pairof the left and right seat rails are joined together to constitute aseat rail assembly, and the seat rail assembly is formed with a box-likeportion disposed at a position connecting a pair of the left and rightsecond attachment portions when the seat rail assembly is viewed fromthe rear.
 17. The motorcycle frame structure according to claim 12,wherein a pair of the left and right seat rails are joined together toconstitute a seat rail assembly, and the seat rail assembly is formedwith a box-like portion disposed at a position connecting a pair of theleft and right second attachment portions when the seat rail assembly isviewed from the rear.
 18. The motorcycle frame structure according toclaim 13, wherein a pair of the left and right seat rails are joinedtogether to constitute a seat rail assembly, and the seat rail assemblyis formed with a box-like portion disposed at a position connecting apair of the left and right second attachment portions when the seat railassembly is viewed from the rear.